Is Matthew 26:32 and John 20:9 a mistake stating that Jesus told the disciples He would rise again then stating they didn't know after it happened?

You are mixing chalk with cheese a bit here, two different things.

"For as yet they knew not the Scripture that he must rise again from the dead." John 20:9.

When it says "they knew not the scripture" it is talking about a verse or verses from the Old Testament, such as Psalm 16:10. They did not yet know that the resurrection is what the Old Testament taught.

So when they were taught about the coming Messiah in their synagogues they were taught he would be a conquering Jewish hero who would drive out the hated Romans and make Judea a great nation again. There was no belief of a Messiah who would die and rise again; their idea of a Messiah was material not spiritual. He wasn't going to come and save from SIN and bring spiritual freedom, they thought he would come to save merely from political oppression. (And of course all the Jews thought that the physical Jews were, all of them, the people of God, and were all going to receive the Messianic benefits when he came. So, for them, the Kingdom of God/Heaven would be the non-spiritual kingdom of the Jews restored to glory by military success.)

So whenever Jesus told them of his death and resurrection they simply didn't get it, it didn't fit their false understanding of the teaching of the Old Testament concerning the coming Messiah.

In the Old Testament the resurrection is taught 1 explicitly; 2 by implication; and 3 figuratively by types and shadows.

1 Explicitly: Psalm 16:10; Psalm 110:1 and following: if he will sit next to God until he makes his enemies a foot stool then he must sit there before (and not merely after) the Day of Judgement. So he must be raised from the dead BEFORE the general resurrection of all mankind.

2 By implication: 2 Samuel 7:12,13 - if he will reign for ever then he must live for ever. He was dead long enough only to be sure he had in fact died, and then rose the third day to reign.

3 And figuratively by types and shadows:-

The Old Testament speaks of Enoch and Elijah both going bodily and alive into heaven. If Enoch ascended alive and bodily into heaven "because he walked with God" (though a sinner) then surely how much more will the coming Messiah ascend bodily and alive into heaven, because he will walk perfectly with God. Elijah's (bodily, alive) departure should tell the same message: Christ would ascend bodily and alive into heaven.

Jonah 1:17 and 2:10 - Jonah "died" and "rose again" and saved the very sinful Ninevites (who Jonah really didn't want to be saved because they were so bad); and the reality is none of us are worth bothering with either. But that reality didn't stop God rescuing them upon their repentance, or us upon our repentance.

Esther 4:16 - Esther was willing to die: she fasted three days and nights, risked death, after this she lived, interceded for the people of God and saved them. The King was not interested in the worthiness of the Jews, but only in the beauty of Esther: it is the same with our salvation, the beauty of Christ, not our own, saves us.

1 Samuel 30:12,13 - David was brought into a desperate state: he gained deliverance not from his own strength or moral uprightness but from an unexpected source: from a suffering servant who was despised and rejected, who "died" and revived, and who gave David and his men victory against their enemies. The Gospel way of salvation is unexpected and incredible to the natural man... salvation by the death and resurrection of another.

Joseph in Egypt figuratively died (e.g. in his brothers plans (Gen 37:20); in the well (Gen 37:24); in his father's eyes (Gen 37:33); in the prison (Gen 39:20) and yet was raised to "life" to be second only to Pharaoh in authority and to use his position to save life, and to save the Children of Israel (Gen 50:20).

Isaac whom his Father was willing to sacrifice at Mount Moriah (Gen 22:2) which Abraham the third day (Gen 22:4) saw afar off, and which would one day be the place of Solomon's Temple in Jerusalem (2 Chron 3:1). This Isaac was figuratively killed and rose again, because a son of Abraham would one day be a sacrifice and die for our sins, rise again, and be the Saviour of the world.

Adam, that is the First Adam, gained his bride Eve by a figurative death and resurrection to foreshadow this would be the means by which the Second Adam, the Lord Jesus Christ, would gain his bride, the Church. (Genesis 2:18-23)

See also Hosea 6:1-3, and Isaiah 53:10-12.

After his resurrection Jesus was at pains to show his disciples that his resurrection was predicted in the Old Testament (Luke 24:25-27) which speaks not only of the Messiah's death, but also of his "entering into his glory" (Luke 24:26).

Jesus's resurrection did not happen out of the blue, the resurrection was part of the prophecy of the Old Testament concerning the coming Messiah. It isn't just a bunch of disciples who bear witness to the resurrection or even "just" 500 disciples (1 Corinthians 15:6) - some extremely important passages of the Old Testament (such as 2 Samuel 7; Psalm 110) predict that when the Messiah comes he will rise from the dead. The promise given to David in 2 Samuel 7 shows that the resurrection and eternal reign as King of one of his descendants was the ultimate objective of God's plan when he made the universe.

The context of Mt. 26:32 is really about Peter and the others abandoning Jesus at the arrest. The shephard will be struck and the sheep scattered. Yes, Jesus tries to reassure them of His resurrection, but they are more interested in the temporal.

Then saith Jesus unto them, All ye shall be offended because of me this night: for it is written, I will smite the shepherd, and the sheep of the flock shall be scattered abroad. But after I am risen again, I will go before you into Galilee. Mat 26:31-32

Jesus won't die, they thought, let alone be resurrected.

With that in mind, John 20:9 is clear; it is simply a reiteration. They didn't believe He would rise from the dead. They see the burial clothes, but don't yet believe. Some translations will use that word "yet" to reiforce that they were told, but didn't believe it.

In addition, the context of John 20:9 is Mary telling them that Jesus' body was missing.

Then she runneth, and cometh to Simon Peter, and to the other disciple, whom Jesus loved, and saith unto them, They have taken away the Lord out of the sepulchre, and we know not where they have laid him. John 20:2

They run to the tomb and find it empty. With great irony and hints of the Garden, scripture says they believed her.

Then cometh Simon Peter following him, and went into the sepulchre, and seeth the linen clothes lie, And the napkin, that was about his head, not lying with the linen clothes, but wrapped together in a place by itself. Then went in also that other disciple, which came first to the sepulchre, and he saw, and believed. John 20:6-8

They believed her, not Him.

For as yet they knew not the scripture, that he must rise again from the dead. John 20:9

So no, there's no contradiction at all, rather confirmation.

The fact that someone is given information does not mean that they know nor understand, nor realise its significance. This is a perfect example.

John chooses the verbs here rather carefully. Koine Greek has two words (verbs) that are translated "know", namely, (a) ginosko, and (b) oida. W E Vine (Expository Dictionary of New Testament Words) comments on the difference in the shades of meaning between these two words: "ginosko frequently suggest inception or progress in knowledge, while oida suggests fullness of knowledge".

In John 20:29, John uses the verb "oida" meaning that while the disciples may have known (been told), they did not fully realise the significance of what Jesus had said. This is yet another reminder that the disciples were very human.